This invention relates to a catalytic partial oxidation process wherein a light hydrocarbon, e.g., methane, is converted to synthesis gas, carbon monoxide and hydrogen. More particularly, this invention relates to a particular particulate catalyst for the catalytic partial oxidation process.
Catalytic partial oxidation is a known process herein a light hydrocarbon, for example, a C1-C4 alkane or hydrocarbon, or more likely methane, as may be in or obtained from natural gas is converted catalytically in the presence of an oxygen containing stream to synthesis gas. The following stoichiometric equation exemplifies the reaction:
CH4+xc2xdO2xe2x86x922H2+CO 
The reaction is particularly attractive in gas to liquids projects wherein natural gas in converted to synthesis gas, and the synthesis gas is converted to heavy hydrocarbons, C2+, via the Fischer-Tropsch process. Because the stoichiometric reactant ratio for the Fischer-Tropsch process with non-shifting catalysts is about 2.1/1, the synthesis gas produced by catalytic partial oxidation is a particularly valuable feed for the Fischer-Tropsch process.
The catalytic partial oxidation process has been reported in a number of recently published patent applications, e.g., EP 0 576 096 A2. Nevertheless, there is a desire to improve both the yield and selectivity of the process, particularly regarding hydrogen selectivity, and thereby further the commercial prospects for the process.
The invention may be exemplified by a catalytic partial oxidation process comprising the reaction of a light hydrocarbon, e.g., a C1-C4 alkyl, preferably methane, by itself, or as a component of natural gas, with oxygen in the presence of a supported, Group VIII noble or non-noble metal catalyst, the support comprising particulate solids of a particular size range.
The prior art in the area of catalytic partial oxidation does not suggest that a particular particle size range exists in which the process becomes highly efficient with regard to hydrogen selectivity. The prior art suggests that while particulates may be used din the catalytic partial oxidation process, it is preferred to use monoliths or foams as the catalyst support. The reasoning being that foams, for example, readily conduct heat because of their bi-continuous structure, whereas particles only conduct heat at narrow point contacts, and therefore, are thought to have a lower overall axial thermal conductivity than foams or monoliths.
Nevertheless, the efficiency of the process and an aspect of this invention is the adequate management of both the heat conductivity, or heat flux along the catalyst bed and the number of active catalytic sites that can be placed on the external surface of the support per unit volume of the reactor.
Due to their large porosity, catalytic support materials, such as foams and monoliths have relatively poorer axial and radial heat conductivity than smaller particulates. Also, the number of active catalytic sites that can be placed on a support is proportional to the surface to volume ratio (S/V), i.e., the external, geometric surface to volume ratio of a granular material, excluding the intra particle surface area, and therefore, smaller and smaller particles would seem to be preferred. However, very small particles do not lend themselves to good heat conductivity in the bed, heat being transferred by point contacts and by radiation along in the catalyst bed. Consequently, there is a need to balance the competing aspects of the overall axial thermal conductivity of the bed with the surface density of catalytic sites in order to achieve the necessary process efficiency.
The surface area to volume (S/V) ratio (also referred to as the geometric surface to volume ratio) of a catalytic bed, e.g., a packed bed, can be readily determined from a knowledge of the particle size and the porosity of the particulate bed. For example, the S/V ratio of a packed bed of spherical particles or particles that can be assumed to be, or described as spherical, can be described by S/V=3(1xe2x88x92Ø)/rp; where Ø is the porosity of the bed and rp is the particle radius. Because packed beds can contain a range of particle sizes, the particle radius can be selected as the radius of the average particle size (i.e., volumetric average particle size). For packed beds, Ø ranges from about 0.3 to about 0.5 and the preferred surface to volume ratio is about 15-230 cmxe2x88x921. However, a more preferred surface to volume range is 18-140 cmxe2x88x921, more preferably 18-105 cmxe2x88x921.
Preferred particles have a diameter ranging from about 200-2000 microns, more preferably about 400-1600 microns, and still more preferably about 400-1200 microns. The particles may be spherical or other shapes which can be described or approximated by a diameter and are generally described as being granular.
Thus, the process balances surface to area ratio, as that ratio has been defined, with the overall bed (or packing) thermal conductivity which is described in D. Kunii and J. M. Smith, AlChE J. 6(1), p. 71-78 (1960), incorporated herein by reference.
The catalyst support is generally a difficult to reduce refractory metal oxide, such as alumina, particularly alpha alumina, zirconia, titania, hafnia, silica, silica-alumina; rare earth modified refractory metal oxides, where the rare earth may be any rare earth metal, e.g., lanthanum, yttrium; alkali earth metal modified refractory metal oxides; and these materials may be generally categorized as materials having a substantially stable surface area at reaction conditions, for example, a surface area that is not substantially altered by reaction conditions, or altered in any way that affects the reaction.
Because thermal conductivity is one of the competing elements for the nature of the catalyst support, the support having better thermal conductivity can be used in the form of smaller average particle sizes. Nevertheless, the S/V ratios given above generally take this element into account and apply for all preferred supports, i.e., zirconia and alpha alumina, particularly preferred being alpha alumina, and rare earth stabilized alumina. The preferred support particles generally have a low total surface area, e.g.,  less than 20 m2/gm, and microporosity is not important to the process.
The catalytic metal is preferably a Group VIII noble metal, e.g., platinum, iridium, rhodium, osmium, ruthenium, although nickel may also be used as the catalytic metal. Rhodium, however, is most preferred as the catalytic metal.
The hydrocarbon feed is preferably a light alkane, e.g., C1-C4, most preferably methane or a gas containing substantial amounts of methane, e.g., natural gas.
The oxygen used in the catalytic partial oxidation process may be pure or substantially pure oxygen or an oxygen containing gas, e.g., air, or a mixture of oxygen with an inert gas. Substantially pure oxygen is preferred, and pure oxygen is still more preferred. Optionally, either the hydrocarbon feed or the oxygen stream, or both, may be mixed with steam. When steam is present, the steam to carbon ratio may be about 0 to 2.5, preferably about 0.2 to 1.5.
The ratio of hydrocarbon feed to oxygen in the reaction zone may range from about 0.45 to about 0.75 oxygen to carbon ratio, more preferably 0.45-0.55. There may be some carbon dioxide in the feed, as for example, from recycle gases or as a diluent. Generally, however, virtually no CO2 is consumed, e.g., CO2 conversion in the catalyst bed is less than about 10%, preferably less than about 5%. Consequently, there is essentially no synthesis gas formation via CO2 reforming.
In an embodiment of the invention reaction temperature is achieved quickly at the inlet of the catalyst bed for best results. In a preferred embodiment, the process takes place in a thin reaction zone, e.g., at high (reaction) temperatures, preferably xe2x89xa65 particle diameters from the bed inlet, more preferably xe2x89xa63 particle diameters from the bed inlet. In this thin zone, substantially all of the oxygen is consumed, preferably xe2x89xa790% of the oxygen is consumed, more preferably xe2x89xa795% of the oxygen is consumed in this zone.
The hydrocarbon synthesis process, also generally known as the Fischer-Tropsch process may be exemplified by contacting synthesis gas, hydrogen and carbon monoxide, with a suitable hydrocarbon synthesis catalyst, e.g., iron, cobalt, or ruthenium, iron being a preferred catalyst for low H2/CO ratio synthesis gas, and cobalt and ruthenium, particularly cobalt, being preferred for higher, i.e.,  greater than 1.0, ratios of H2/CO synthesis gas. More preferably, a non-shifting catalyst, e.g., Co, is preferred, more preferably supported cobalt. While any reactor type, fixed bed, fluid bed, slurry bed, may be employed, slurry bubble columns, where injected synthesis gas provides all or at least a portion of the energy required for maintaining the catalyst dispersed (i.e., fluidized) in the bubble column, are preferred. See, for example, U.S. Pat. No. 5,348,982 incorporated herein by reference.
The catalyst can be prepared by any technique, and conventional techniques, e.g., impregnation, incipient wetness, spray drying, etc., and may be exemplified by: placing the particulate support in an aqueous solution of a desired catalytic metal, e.g., rhodium nitrate of appropriate concentration.
To yield a catalyst with suitable metal loadings, the impregnated support is removed from the metal solution, dried (at temperatures of up to about 125xc2x0 C.) to remove moisture, and calcined, usually in air, for example at temperatures of about 300 to 600xc2x0 C. The metal loading is that which is catalytically effective, for example ranging from about 2-25 wt % metal, preferably about 3-20 wt % metal. The oxide is normally quickly reduced to the elemental and active form of the metal in the course of the catalytic partial oxidation process.
The catalytic partial oxidation process is conducted at suitable reaction conditions, such as those described in prior patents and patent applications. Thus, pressures may range from 1-100 atmospheres, and feed is contacted with the catalyst at temperatures ranging from 400-1200xc2x0 C., preferably 500-800xc2x0 C. Typically, gas hourly space velocities may range from a low of about xe2x89xa7300,000 hrxe2x88x921 to about 10,000,000 hrxe2x88x921 or higher, preferably at least about 600,000 hrxe2x88x921, more preferably at least about 1,000,000 hrxe2x88x921, still more preferably at least about 1,300,000 hrxe2x88x921.
The following examples will illustrate this invention, but are not meant to limit, in any way, the invention.
CATALYST PREPARATION
Supports were alumina spheres with diameters of 400, 800, 1200 and 3200 xcexcm. Packed beds were prepared using the spherical particles and the porosity, interparticle pore diameters, and S/V as follows:
Where the interparticle pore diameter is calculated by [2 Ø/(3(1xe2x88x92Ø))][Dp], where Dp is the particle diameter.
Prior to use, the alumina particles were sintered to decrease the internal surface area to  less than 15 m2/gm (i.e., 400 xcexcm:6.9 m2/gm; 800 xcexcm:6.5 m2/gm; 1200 xcexcm:14.5 m2/gm; 3200 xcexcm:12.3 m2/gm.)
Calcination involved heating from 120xc2x0 C. to 1000xc2x0 C. at 20xc2x0 C./minute, 1000xc2x0 C. to 1200xc2x0 C. at 5xc2x0 C./minute; calcine at 1200xc2x0 C. for 6 hours.
Rhodium was applied to the sintered particles by placing them in an aqueous solution of rhodium nitrate of a concentration to obtain the desired metal loadings. The solution containing the particles was dried overnight to remove moisture, and the particles were then calcined at 600xc2x0 C. for 6 hours.
For comparative purposes Rh catalyzed monoliths were also prepared. Two alumina based (92% Al2O3, 8% SiO2) monoliths were used: R1 having 45 ppi (pores per inch) with an average pore diameter of 420 xcexcm, R2 having 80 ppi with an average pore diameter of 210 xcexcm. Surface areas of each were  less than 1 m2/gm.
Rhodium was applied to each monolith using a technique similar to that described for particles. The monoliths were impregnated with rhodium nitrate and left to dry overnight. The procedure was repeated where necessary to obtain the desired rhodium loadings. The monoliths were then calcined at 600xc2x0 C. for 6 hours.
CATALYST TESTING PROCEDURES
Rh catalyzed spheres and monoliths were tested in plug flow quartz reactors. In all tests, Fiberfrax insulation (Fiberfrax Durablanket, Carborundum) covered with aluminum foil, was wrapped around the outside of quartz reactor tubes in order to prevent radial heat losses. Catalyzed spheres were supported on a quartz frit inside the quartz reactor tube. Upstream of the catalyzed spheres a ceramic fiber mat (Nextel 312, a 3M product: 62% Al2O3, 24% SiO2, 14% B2O3) was used to help prevent heat losses. Catalyzed monoliths were sandwiched between two uncatalyzed, blank 45 ppi alumina monoliths (Hi Tech Ceramics: 92% Al2O3, 8% SiO2) to help prevent heat losses. All monoliths were wrapped with Fiberfrax paper (Carborundum) to seal them tightly in the quartz reactor tube.
Feed gases, CH4, O2, and N2, were controlled with Brooks mass flow controllers (5850E flow controllers). Unless specified otherwise, 20% of the feed was N2 and the CH4/O2 ratio was 2 to 1. Feed gases were premixed and fed to the catalyst at ambient temperature. Pressure was maintained at 2 psig. The reactor was ignited by placing a bunsen burner on the reactant tube with feed gases flowing. When the ignition temperature was reached, the bunsen burner was removed and insulation was placed over the quartz tube containing the catalyst as described above. Product gases (CO, H2, CO2, C2H2, C2H4, C2H6) and unconverted feed gases (CH4, O2, N2) were analyzed with an HP 5890, Series II gas chromatograph. All mass balances closed within xc2x15%, with error usually less than 3%.